Focused rotational ivus transducer using single crystal composite material

ABSTRACT

An ultrasound transducer for use in intra-vascular ultrasound (IVUS) imaging systems including a single crystal composite (SCC) layer is provided. The transducer has a front electrode on a side of the SCC layer; and a back electrode on the opposite side of the SCC layer. The SCC layer may have a dish shape including pillars made of a single crystal piezo-electric material embedded in a polymer matrix. Also provided is an ultrasound transducer as above, with the back electrode split into two electrodes electrically decoupled from one another. A method of forming an ultrasound transducer as above is also provided. An IVUS imaging system is provided, including an ultrasound transducer rotationally disposed within an elongate member; an actuator; and a control system controlling activation of the ultrasound transducer to facilitate imaging.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. patent application Ser.No. 14/135,063, filed Dec. 19, 2013, which claims priority to and thebenefit of U.S. Provisional Patent Application No. 61/745,425, filedDec. 21, 2012, which are hereby incorporated by reference in theirentireties.

TECHNICAL FIELD

The present disclosure relates generally to intravascular ultrasound(IVUS) imaging inside the living body and, in particular, to an IVUSimaging catheter that relies on a mechanically-scanned ultrasoundtransducer, including embodiments where the transducer includes a singlecrystal composite material.

BACKGROUND

Intravascular ultrasound (IVUS) imaging is widely used in interventionalcardiology as a diagnostic tool for a diseased vessel, such as anartery, within the human body to determine the need for treatment, toguide the intervention, and/or to assess its effectiveness. IVUS imaginguses ultrasound echoes to create an image of the vessel of interest. Theultrasound waves pass easily through most tissues and blood, but theyare partially reflected from discontinuities arising from tissuestructures (such as the various layers of the vessel wall), red bloodcells, and other features of interest. The IVUS imaging system, which isconnected to the IVUS catheter by way of a patient interface module(PIM), processes the received ultrasound echoes to produce across-sectional image of the vessel where the catheter is placed.

In a typical rotational IVUS catheter, a single ultrasound transducerelement is located at the tip of a flexible driveshaft that spins insidea plastic sheath inserted into the vessel of interest. The transducerelement is oriented such that the ultrasound beam propagates generallyperpendicular to the axis of the catheter. A fluid-filled sheathprotects the vessel tissue from the spinning transducer and driveshaftwhile permitting ultrasound signals to freely propagate from thetransducer into the tissue and back. As the driveshaft rotates(typically at 30 revolutions per second), the transducer is periodicallyexcited with a high voltage pulse to emit a short burst of ultrasound.The same transducer then listens for the returning echoes reflected fromvarious tissue structures, and the IVUS imaging system assembles a twodimensional display of the vessel cross-section from a sequence of thesepulse/acquisition cycles occurring during a single revolution of thetransducer.

In the typical rotational IVUS catheter, the ultrasound transducer is apiezoelectric ceramic element with low electrical impedance capable ofdirectly driving an electrical cable connecting the transducer to theimaging system hardware. In this case, a single pair of electrical leads(or coaxial cable) can be used to carry the transmit pulse from thesystem to the transducer and to carry the received echo signals from thetransducer back to the imaging system by way of a patient interfacemodule (“PIM”) where the echo signals can be assembled into an image. Animportant complication in this electrical interface is how to transportthe electrical signal across a rotating mechanical junction. Since thecatheter driveshaft and transducer are spinning (in order to scan across-section of the artery) and the imaging system hardware isstationary, there must be an electromechanical interface where theelectrical signal traverses the rotating junction. In rotational IVUSimaging systems, this problem can be solved by a variety of differentapproaches, including the use of rotary transformers, slip rings, rotarycapacitors, etc.

While existing IVUS catheters deliver useful diagnostic information,there is a need for enhanced image quality to provide more valuableinsight into the vessel condition. For further improvement in imagequality in rotational IVUS, it is desirable to use a transducer withbroader bandwidth and to incorporate focusing into the transducer. Apiezoelectric micro-machined ultrasound transducer (PMUT) fabricatedusing a polymer piezoelectric material offers greater than 100%bandwidth for optimum resolution in the radial direction, and aspherically-focused aperture for optimum azimuthal and elevationresolution. While this polymer PMUT technology offers many advantages,the electrical impedance of the PMUT is too high to efficiently drivethe electrical cable connecting the transducer to the IVUS imagingsystem by way of the PIM. Furthermore, the transmit efficiency ofpolymer piezoelectric material is much lower compared to that of thetraditional lead-zirconate-titanate (PZT) ceramic piezoelectric.Therefore, the signal-to-noise ratio of a PMUT will be compromisedunless the deficiency in acoustic output can be compensated for byimproved transmit electronics and/or other signal processing advances.

Current approaches to form a focused ultrasound beam include the use ofan acoustic lens using conventional PZT transducers. For example, arubber lens with an acoustic velocity of 1.0 mm/μsec has been used forelevation focus in phased array ultrasound systems. These approachespose complex fabrication problems and the difficulty of removing imagingartifacts in the resulting signal.

Accordingly, there remains a need for improved devices, systems, andmethods for implementing focused piezoelectric micro-machined ultrasonictransducers within an intravascular ultrasound system.

SUMMARY

According to some embodiments, an ultrasound transducer for use inintra-vascular ultrasound (IVUS) imaging systems is provided thatincludes a single crystal composite (SCC) layer; a front electrode on aside of the SCC layer; and a back electrode on the opposite side of theSCC layer. In some embodiments, the SCC layer includes pillars made of asingle crystal piezo-electric material. The pillars are embedded in apolymer matrix in some instances. The SCC layer has a dish shape,defined by a concave surface and opposing convex surface, in someembodiments. The back electrode is split into two electrodeselectrically decoupled from one another in some implementations.

A method of forming an ultrasound transducer for use in IVUS imagingsystems in some embodiments includes etching a single crystal; forming apolymer layer on the etched single crystal to form a single crystalcomposite (SCC) having a first thickness; placing a first electrode on afirst side of the SCC; forming the SCC to a second thickness; placing asecond electrode on a second side of the SCC; and placing the SCC on amolded tip.

An IVUS imaging system according to some embodiments may include anultrasound emitter and receiver rotationally disposed within an elongatemember; an actuator coupled to the ultrasound emitter, the actuatormoving the ultrasound emitter through at least a portion of an arc; anda control system controlling the emission of a sequence of pulses fromthe ultrasound emitter and receiving from the receiver ultrasound echodata associated with the pulses, the control system processing theultrasound echo data to generate a cross-sectional image of the vessel.In some embodiments the ultrasound emitter and receiver comprises anultrasound transducer including a single crystal composite (SCC) layer;a front electrode; and a back electrode. In some embodiments the SCClayer includes pillars made of a single crystal piezo-electric material.The pillars are embedded in a polymer matrix in some instances. The SCClayer has a dish shape, with opposing concave and convex surfaces, insome embodiments.

These and other embodiments of the present disclosure will be describedin further detail below with reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an imaging system according to anembodiment of the present disclosure.

FIG. 2 is a diagrammatic, partial cutaway perspective view of an imagingdevice according to an embodiment of the present disclosure.

FIG. 3 shows a partial view of an ultrasound transducer according to anembodiment of the present disclosure.

FIG. 4 shows a partial cross-sectional side view of a distal portion ofan imaging device according embodiment of the present disclosure.

FIG. 5A shows a partial cross-sectional axial view of the distal portionof the imaging device of FIG. 4 along section line A-A′.

FIG. 5B shows a partial cross-sectional axial view of the distal portionof the imaging device of FIG. 4 along section line B-B′.

FIG. 6A shows a partial plan view of a single crystal compositeaccording to an embodiment of the present disclosure.

FIG. 6B shows a partial plan view of a single crystal compositeaccording to another embodiment of the present disclosure.

FIG. 6C shows a partial plan view of a single crystal compositeaccording to yet another embodiment of the present disclosure.

FIG. 7A shows a partial side view of an ultrasound transducer accordingto an embodiment of the present disclosure.

FIG. 7B shows a partial plan view of a distal portion of an imagingdevice incorporating the ultrasound transducer of FIG. 7A according toan embodiment of the present disclosure.

FIG. 7C shows a partial plan view of the ultrasound transducer of FIG.7A according to an embodiment of the present disclosure.

FIGS. 8A-F show a series of partial cross-sectional side views offabrication stages for an ultrasound transducer according to someembodiments of the present disclosure.

FIG. 9 shows a flow chart for a method of forming an ultrasoundtransducer according to some embodiments of the present disclosure.

In the figures, elements having the same reference number have the sameor similar functions and/or features.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It is nevertheless understood that no limitation tothe scope of the disclosure is intended. Any alterations and furthermodifications to the described devices, systems, and methods, and anyfurther application of the principles of the present disclosure arefully contemplated and included within the present disclosure as wouldnormally occur to one skilled in the art to which the disclosurerelates. In particular, it is fully contemplated that the features,components, and/or steps described with respect to one embodiment may becombined with the features, components, and/or steps described withrespect to other embodiments of the present disclosure. For the sake ofbrevity, however, the numerous iterations of these combinations will notbe described separately.

Embodiments disclosed herein are for an apparatus and a method offabrication of the apparatus, the apparatus including a focusedtransducer to be used in a rotational IVUS catheter. A transducer asdisclosed herein provides a broad bandwidth of ultrasound signals havingfocused beam propagation. Such an ultrasound beam provides a highthree-dimensional (3D) resolution for ultra-sound imaging, includingdepth, lateral and elevation dimensions. In some embodiments, an IVUScatheter of the present disclosure provides a wide bandwidth, focusedultrasound beam without increasing the number of electrical connectionsto a circuit rotating together with the transducer. An ultrasoundtransducer according to embodiments disclosed herein may include asingle crystal composite material that provides a wide bandwidth,focused beam. The single crystal composite material is shaped into anelement having a curvature designed to provide a focused beam (e.g.,defining a concave emitting surface for the ultrasound transducer) insome instances.

FIG. 1 shows an IVUS imaging system 100 according to an embodiment ofthe present disclosure. In some embodiments of the present disclosure,the IVUS imaging system 100 is a rotational IVUS imaging system. In thatregard, the main components of the rotational IVUS imaging system are arotational IVUS catheter 102, a patient interface module (PIM) 104, anIVUS console or processing system 106, and a monitor 108 to display theIVUS images generated by the IVUS console 106. Catheter 102 includes anultrasound transducer 150 in some embodiments. PIM 104 implements theappropriate interface specifications to support catheter 102. Accordingto some embodiments, PIM 104 generates a sequence of transmit pulsesignals and control waveforms to regulate the operation of ultrasoundtransducer 150. PIM 104 may also receive a response signal formtransducer 150 through the same pair of lines.

Ultrasound transducer 150 transmits ultrasound signals towards thevessel tissue based on the trigger signals received from PIM 104.Ultrasound transducer 150 also converts echo signals received from thevessel tissue into electrical signals that are communicated to PIM 104.PIM 104 also supplies high- and low-voltage DC power supplies to therotational IVUS catheter 102. In some embodiments, PIM 104 delivers a DCvoltage to transducer 150 across a rotational interface. Options fordelivering DC power across a rotating interface include the use ofslip-rings, rotary transformers, and/or the implementation of the activespinner technology.

FIG. 2 shows a diagrammatic, partial cutaway perspective view ofcatheter 102, according to an embodiment of the present disclosure. FIG.2 shows additional detail regarding rotational IVUS catheter 102.Rotational catheter 102 includes an imaging core 110 and an outercatheter/sheath assembly 112. Imaging core 110 includes a flexible driveshaft that is terminated at the proximal end by a rotational interface114 providing electrical and mechanical coupling to PIM 104 (cf. FIG.1). The distal end of the flexible drive shaft of the imaging core 110is coupled to a transducer housing 116 containing ultrasound transducer150 and associated circuitry.

Catheter/sheath assembly 112 includes a hub 118 supporting rotationalinterface 114 and provides a bearing surface and a fluid seal betweenrotating and non-rotating elements of catheter 102. In some embodiments,hub 118 includes a luer lock flush port 120 through which saline isinjected to flush out the air and fill the inner lumen of the sheathwith an ultrasound-compatible fluid at the time of use of the catheter.Saline also provides a biocompatible lubricant for the rotatingdriveshaft. In some implementations, hub 118 is coupled to a telescope122 that includes nested tubular elements and a sliding fluid seal thatpermits catheter/sheath assembly 112 to be lengthened or shortened.Telescope 122 facilitates axial movement of the transducer housingwithin an acoustically transparent window 124 at the distal portion ofcatheter 102.

In some embodiments, window 124 is composed of thin-walled plastictubing fabricated from material(s) that readily conduct ultrasound wavesbetween the transducer and the vessel tissue with minimal attenuation,reflection, or refraction. A proximal shaft 126 of catheter/sheathassembly 112 bridges the segment between telescope 122 and window 124.In some embodiments, proximal shaft 126 is composed of a material orcomposite that provides a lubricious internal lumen and optimumstiffness to catheter 102. Embodiments of window 124 and proximal shaft126 in catheter 102 may be as described in detail in US patentapplication entitled “Intravascular Ultrasound Catheter for MinimizingImage Distortion,” U.S. Patent Application No. 61/746,958 filed Dec. 28,2012, Attorney docket No. 44755.938, now published as U.S. PatentApplication Publication No. 2014/0187964 A1 on Jul. 13, 2014, thecontents of which are hereby incorporated in their entirety byreference, for all purposes.

FIG. 3 shows a partial view of ultrasound transducer 150 according tosome embodiments disclosed herein. Transducer 150 includes a singlecrystal composite material (SCC) 301 having pillars 320 of a singlecrystal piezo-electric material embedded in a polymer matrix 330. Insome embodiments polymer matrix 330 is formed by epoxy. The epoxy usedas filler in polymer matrix 330 provides flexibility to the SCC materialforming ultrasound transducer 150.

In some embodiments, an impedance matching layer 310 is included inultrasound transducer 150. Impedance matching layer 310 facilitatescoupling of the acoustic wave with the medium surrounding ultrasoundtransducer 150. Soft polymer matrix 330 reduces the acoustic impedanceto SCC 301, thus providing high efficiency and broad bandwidth totransducer 150 for acoustic coupling. In some embodiments, matchinglayer 310 may be a quarter-wave matching layer added to SCC 301, tofurther improve efficiency and bandwidth of transducer 150, thusenhancing sensitivity.

According to some embodiments disclosed herein, pillars 320 formstructures elongated in an axial direction (Y-axis in FIG. 3) having anarrow diameter in cross section (Z-axis in FIG. 3). The cross sectionof pillars 320 is in a plane of SCC 301 forming ultrasound transducer150. Further according to some embodiments, polymer matrix 330 iscontinuous in the axial direction (Y-axis) and in the plane of SCC 301forming ultrasound transducer 150 (XZ-plane). The anisotropic nature ofSCC 301 confines the electric field E within pillars 320, which have ahigh dielectric constant. Fringe fields at the edges of electrodes 151and 152 are mitigated by polymer matrix 330. Thus, in some embodimentsthe performance of transducer 150 is not degraded by fringe fields atelectrode boundaries. According to some embodiments, the thickness (orheight) of pillars 320 may be 50 μm or less, for 40 MHz center frequencyoperation. In some embodiments a thickness-to-width aspect ratio of atleast 2 or greater may be desirable, resulting in pillars 320 having adiameter of 20 μm or less.

Accordingly, SCC 301 can be made using deep reactive ion etching (DRIE)applied to a single crystal material. Etch a matrix pattern using DRIAand fill the etched trenches with epoxy. Then grind away back side andpolish front side and have a resulting composite layer. A horizontalresonant frequency (oscillations in the XZ plane in FIG. 3) is so farapart from vertical frequency (oscillations along the Y-axis in FIG. 3)there is little energy expended by horizontal resonance. This makes thetransducer more efficient. Wide bandwidth is achieved by efficientlycoupling into medium (for example, using a matching layer). A matchinglayer overcomes acoustic impedance mismatch between transducer materialand the transmitting medium. A PZT has impedance of about 30 while thatof blood/saline solutions is about 1.6/1.5. A matching layer allows thetransition from the PZT material to the transmitting medium moreefficient. In some embodiments, the epoxy used to form SCC 301 may beused as an impedance matching layer. The impedance of epoxy is about 3,while impedance of SCC 301 depends on distribution of PZT ceramicpillars within the epoxy matrix. In some embodiments the acousticimpedance of SCC 301 may be approximately 10. Adding a matching layerand/or a backing material to transducer 150 increases the bandwidth. Theshape of pillars 320 may lightly impact the device bandwidth and centerfrequency of operation. Acoustic loss of the epoxy matrix affects thebandwidth of transducer 150. The epoxy serves to absorb or dissipatesound. Any energy that attempts to stay in the plastic will be absorbedquickly. An added advantage of SCC 301 is the higher electric fielddensity in pillars 320 relative to epoxy matrix 330 due to the higherdielectric constant of the PZT ceramic relative to the epoxy. Thisincreases the coupling efficiency of the transducer.

A piezoelectric material typically has a 20:1 acoustic impedancemismatch with blood and saline. A composite material increases theproportion of epoxy and polymers in transducer 150, reducing acousticimpedance and providing better impedance matching. Bandwidth may beimproved by including a backing material overlaid on transducer 150 toabsorb acoustic energy, increasing bandwidth at the cost of somewhatreduced signal strength.

SCC 301 provides high efficiency and broad bandwidth for ultrasoundgeneration and sensing, which is desirable in medical applications.According to some embodiments, single crystal piezoelectric materialsused in SCC 301 have a high electromechanical coupling coefficient. Theelectromechanical coupling coefficient of single crystal piezoelectricmaterials is typically higher than PZT ceramic. Thus, the voltage levelsneeded for a predetermined volume change is lower for the single crystalmaterials used in SCC 301, relative to that of piezo-electric ceramics.This increases the power conversion efficiency of SCC 301 fromradiofrequency energy into sound, and from sound into radiofrequencyenergy. Some embodiments include narrow pillars 320 that remove thelateral constraint on the piezoelectric material that is present in acontinuous slab of material. The lateral constraint of a bulk crystal isrelated to the rigidity of the material, as the pillars embedded inepoxy stretch longitudinally, there is less resistance from thesurrounding epoxy material since the epoxy material is less rigid. Insuch embodiments, low frequency lateral modes (in the XZ-plane in FIG.3) in the vicinity of the desired ultrasound frequency are suppressed innarrow pillars 320 by the surrounding polymer matrix 330. Thus, most ofthe RF electrical energy in SCC 301 is transferred to ‘height’ vibrationmodes (Y-axis in FIG. 3) in pillars 320, which couple to the ultrasoundwaves forming the probe beam. In some embodiments, polymer matrix 330reduces the acoustic impedance of SCC 301 compared to that of a singlecrystal material. Indeed, the young modulus of polymer matrix 330 islower than that of the single crystal 320, or that of a piezo-electricceramic. For example, in some embodiments SCC 301 may include 75% involume of polymer matrix 330. Such a composite has low acousticimpedance compared to a slab of single crystal piezoelectric orpiezo-ceramic material. This low acoustic impedance is better matched totissue acoustic impedance, therefore providing high efficiency and broadbandwidth to SCC 301.

The dimensions of SCC 301 vary according to the specific applicationsought. For example, the target ultrasound frequency and bandwidthdetermine the specific dimensions of SCC 301 in some instances. In someembodiments pillars 320 are about 10 μm in diameter (Z-axis in FIG. 3)with 10 μm deep kerfs (pillar height, Y-axis in FIG. 3). For highfrequency IVUS, it may be desirable to have even smaller structures andkerfs in SCC 301. In some embodiments single crystal materials may bedesirable in SCC 301 for high frequency applications because singlecrystals may be patterned using deep reactive ion etching (DRIE). DRIEtechniques may be used to pattern the crystalline substrate with micronaccuracy to fabricate SCC 301 materials on a wafer scale.

The volume fraction of polymer matrix 330 in SCC 301 may also varyaccording to the specific application. For example, the volume fractionof polymer matrix 330 determines the impedance of the transducermaterial which is beneficial to match acoustical impedance of the tissueof interest for the use of the ultrasound beam in some instances. Thethickness of the composite crystal is determined by the resonancefrequency desired. The thickness of SCC 301 is chosen to obtain apre-selected center frequency of a transmitted ultrasound signal fromtransducer 150.

FIG. 4 shows a partial perspective view of transducer housing 116,including ultrasound transducer 150 according to some embodiments.Ultrasound transducer 150 includes SCC 301 and impedance matching layer310. Other details of ultrasound transducer 150 are omitted in FIG. 4,for clarity. It is understood that ultrasound transducer 150 in FIG. 4may include the same or similar elements as shown in FIG. 3. Forexample, ultrasound transducer 150 in FIG. 4 may include back electrode151, and front electrode 152.

In some embodiments SCC 301 is deformed into a curved shape. Forexample, SCC 301 is deformed into a dish-shaped structure having asymmetry axis included in a plane that also includes the Z-axis in FIG.3. In some embodiments, the dish-shaped structure may be symmetric aboutthe BD axis, which may be parallel to the Y-axis, or may be forming anangle relative to the Y-axis. This may be desirable for providing afocused ultrasound beam. For example, in some instances SCC 301 isdeformed such that the upper surface of SCC 301 as viewed in FIG. 3becomes concave. In some implementations, the concave shape of the uppersurface of SCC 301 is generally spherical. Further, in some instancesthe deformation of SCC 301 results in the lower surface of SCC 301 beingconvex. In some implementations, the convex shape of the lower surfaceof SCC 301 is generally spherical. In some particular implementations,the concave upper surface and the convex lower surface are bothgenerally spherical with a common center point.

Transducer housing 116 is in a distal portion of catheter 102 accordingto an embodiment of the present disclosure. In particular, FIG. 4 showsan expanded view of aspects of the distal portion of imaging core 110.In this exemplary embodiment, imaging core 110 is terminated at itsdistal tip by housing 116. Housing 116 may be fabricated from stainlesssteel or other suitable biocompatible material, and have a bullet-shapedor rounded nose, and an aperture 128 for an ultrasound beam. Thus,ultrasound beam 130 may emerge from housing 116, through aperture 128.In some embodiments, flexible driveshaft 132 of imaging core 110 iscomposed of two or more layers of counter wound stainless steel wires.Flexible driveshaft 132 is welded or otherwise secured to housing 116such that rotation of flexible driveshaft 132 also imparts rotation tohousing 116. In the illustrated embodiment, an electrical cable 134delivers the high-voltage transmit pulse and carries the low amplitudeecho signal back to PIM 104. with an optional shield 136 provideselectrical power to SCC 301. Electrical cable 134 extends through aninner lumen of flexible driveshaft 132 to the proximal end of imagingcore 110 where it is terminated to the electrical connector portion ofthe rotational interface 114 (cf. FIG. 2). SCC 301 is mounted ontomolded tip 148. Molded tip 148 may be formed of a polymer material suchas epoxy, and serve as an acoustic backing material to absorb acousticreverberations propagating within housing 116. Molded tip 148 providesstrain relief for electrical cable 134 at the point of soldering toelectrodes 151 and 152 in some instances. In some embodiments, aflexible sheet of material is molded into bowl shaped substrate to havea concave shape.

According to some embodiments, molded tip 148 is formed such that anupper surface of the molded tip is concave so that when ultrasoundtransducer 150 is placed on the concave upper surface, the flexibilityof SCC 301 allows ultrasound transducer 150 to acquire a correspondingcurved shape. In some instances, a bottom surface of the ultrasoundtransducer 150 matches the curvature of the upper surface of the moldedtip 148. Accordingly, in some such instances the bottom surface of theultrasound transducer 150 becomes convex and an opposing upper surfaceof the ultrasound transducer becomes concave (as shown in FIGS. 4 and5B). The convex shape of the lower surface of the ultrasound transducer150 may have an apex along the axis of beam direction BD such that atangent to apex of the surface forms an angle θ with a longitudinal axisof catheter 102 (Z-direction in FIG. 4). In that regard, in someembodiments the concave upper surface of the ultrasound transducer 150is symmetrical about the axis of beam direction BD such that ultrasoundbeam 130 emitted from the ultrasound transducer 150 propagates alongdirection BD into the vessel tissue. FIG. 4 shows a BD substantiallyorthogonal to the longitudinal axis of catheter 102 (θ˜90°). One ofordinary skill will recognize that angle θ may have values smaller than90° or larger than 90°, depending on the desired features for ultrasounddata processing. In that regard, in some implementations, the ultrasoundtransducer 150 is mounted such that the ultrasound beam 130 propagatesat an oblique angle with respect to the longitudinal axis of thecatheter.

The curvature adopted by ultrasound transducer 150 according toembodiments as disclosed herein provides focusing for beam 130. In someembodiments aperture 128 may be about 500 μm in diameter (d), and afocal length, f:3d, may be desired to obtain sufficient resolution anddepth of field. Thus, the geometric focus of ultrasound beam 130 may beabout 1 mm outside the sheath (1.5 mm from the aperture). For a curvedtransducer of this geometry, the depth of the dish should beapproximately 20 μm. In some embodiments, the wavelength of a centerfrequency of an ultrasound signal transmitted by transducer 150 is about40 μm in the transducer material. Accordingly, the diameter of thetransducer may fit about a ten, a dozen, or a similar number ofwavelengths within its surface.

In some embodiments an acoustic lens may be used to provide focusing tobeam 130. To achieve a lens, some embodiments may use silicone or someother polymer that reduces sound speed through the lens materialrelative to that of the medium. For example, ultrasound waves may travelat a 1.0 mm/μsec velocity in a silicone lens, versus 1.5 mm/μsec mediumvelocity. This may provide a similar focusing power (f:3d) to the 20 μmdeep dish described above with a lens thickness of approximately 60 μm.Such a lens may be formed by surface tension under a microscope, tocontrol thickness. For example a lens may be formed with a glue drophaving a concavity provided by surface tension. A material may be assilicon rubber (slow material). But careful with losses.

The curved transducer approach as shown in FIG. 4 facilitates mitigatingreflections, reverberation, attenuation, and other diffraction effectsresulting from using refractive elements in the path of ultrasound beam130 in some embodiments.

FIG. 5A shows a partial cross-section view of a transducer housingincluding electrical leads 134-1 and 134-2, according to someembodiments. FIG. 5A results from taking a cut away view of FIG. 4 alongline AA′. Electrical leads 134-1 and 134-2 may be collectively referredto as leads 134 (cf. FIG. 4). Leads 134 may be coupled to bonding pad506 (lead 134-1) and to bonding pad 507 (lead 134-2). Bonding pads 506and 507 may have electrical contact with either of electrodes 151 and152 in ultrasound transducer 150. In some instances, electric leads134-1 and 134-2 provide a high and a low voltage signal coupled to SCC301 through electrodes 151 and 152. In some embodiments lead 134-1 iscoupled to back electrode 151 and lead 134-2 is coupled to frontelectrode 152. Further, according to some embodiments leads 134-1 and134-2 are coupled to different portions of back electrode 151. In suchconfigurations, front electrode 152 may have a floating voltage having avalue between the voltages provided by leads 134-1 and 134-2.Embodiments having a floating electrode 152 may reduce the connectionsused inside housing 116. In particular, embodiments having a floatingelectrode 152 may enable the use of a continuous index matching layer310.

FIG. 5B shows a partial cross-section view of transducer housing 116,including ultrasound transducer 150, according to some embodiments. FIG.5B results from taking a cut away view of FIG. 4 along line BB′. FIG. 5Billustrates aperture 128 formed above ultrasound transducer 150 to allowultrasound beam 130 to pass through, into and from the vessel tissue.FIG. 5B also shows window 124, which is transparent to the ultrasoundbeam 130 coupling transducer 150 with the vessel tissue (cf. FIG. 2).

FIGS. 6A, 6B, and 6C show partial plan views of single crystalcomposites 601A, 601B, and 601C, respectively, according to embodimentsdisclosed herein. Without loss of generality, SCC 601A, SCC 601B, andSCC 601C in FIGS. 6A, 6B, and 6C are shown in a plane XZ consistent withCartesian coordinate axes shown in FIGS. 1-5B. One of ordinary skill inthe art will recognize that an ultrasound transducer fabricated from anyone of SCC 601A, 601B, and 601C may have any orientation in 3D space. Inparticular, as has been discussed above, an ultrasound transducer formedfrom SCC 601A, SCC 601B, and SCC 601C may have a 3D curvature forming adish shape having a symmetry axis, BD, as shown in FIG. 4. SCC 601A, SCC601B, and SCC 601C (collectively referred to as SCC 601) include pillars620A, 620B, and 620C, respectively (collectively referred to as pillars620). Pillars 620 in SCC 601 are embedded in polymer matrix 630. In someembodiments polymer matrix 630 may be as polymer matrix 330, describedin detail with reference to FIG. 3, above. Also illustrated in FIGS. 6A,6B, and 6C is a cutout path 650 in the XZ plane. Cutout path 650 may beformed with a laser beam on portions of SCC 601 including polymer matrix630.

One of ordinary skill will recognize that the portion of the total areaof SCC 601A, 601B, and 601C covered by pillars 620A, 620B, and 620C mayvary. In some embodiments pillars 620A, 620B, and 620C may cover an areaof about 25% of a surface area of SCC layer 601A, 601B, and 601C,respectively.

As shown in FIG. 6A, SCC 601A includes pillars 620A having a circularcross-section in the XZ plane. As shown in FIG. 6B, SCC 601B includespillars 620B having a square cross-section in the XZ plane. As shown inFIG. 6C, SCC 601C includes pillars 620C having puzzle-piececross-section in the XZ plane. One of ordinary skill would recognizethat the particular shape of pillars in SCC 601 in the XZ plane is notlimiting. Some embodiments may include pillars having cross-sections inthe XZ plane with dog-bone shape, pseudo-random shape, and hexagonalshape.

Embodiments such as SCC 601A, 601B, 601C, or similar non-traditionalshapes provide improved fill efficiency in the XZ plane, improvedadhesion to polymer matrix 630, greater flexibility, and bettersuppression of undesired lateral modes (in the XZ plane). Furthermore,SCC 601 provides improved mechanical integrity during the wafer thinningprocess. Patterning the finished transducer with cutout path 650 is alsoa valuable benefit. In some embodiments, cutout path 650 may form acircular or elliptical transducer shape. Ultrasound transducers havingcircular or elliptical shapes offer good performance in terms ofside-lobe levels, compared to cutout paths having rectangular or squareshapes.

The geometrical configuration of pillars 620 shown in FIG. 6 is notlimiting to patterns 620A, 620B, or 620C. One of ordinary skill willrecognize that many configurations are possible. In some embodiments theaperture formed by SCC 601 may be apodized by adjusting the density ofpillars 620 near the edges of the aperture (close to cutout path 650) tofurther reduce side-lobe levels. Some embodiments include pillars 620having cross-sections with shapes obtained from Escher styletessellations of XZ-plane. In some embodiments, odd-shaped but uniformpillars 620 are used.

FIG. 7A shows a partial side view of an ultrasound transducer 750according to some embodiments disclosed herein. Embodiments of splitback electrode transducer 750 include a back electrode divided into twoequal halves 751-1 and 751-2. In some embodiments, halves 751-1 and751-2 have a D-shape where the transducer has a circular or ellipticalprofile. Halves 751-1 and 751-2 are electrically decoupled from oneanother, so that each half may be coupled to a different voltage. Thefront electrode is continuous over the entire front surface oftransducer 750 in some instances. In ultrasound transducer 750 theelectrode connections to electrical cables 734-1 and 734-2 are providedfrom the back side. Thus, the back electrode in ultrasound transducer750 includes back side portion 751-1 connected to cable 734-1, and backside portion 751-2 connected to cable 734-2. According to someembodiments, front electrode 752 may float with no direct contact to anoutside voltage source, or ground. Ultrasound transducer 750 includesSCC 701, which may include single crystal pillars embedded in a polymersimilar to SCC 301 and SCC 601 as described in detail above (cf. FIGS.1, 6A, 6B, and 6C).

Some embodiments of ultrasound transducer 750 with a split backelectrode configuration as in FIG. 7A include SCC 701 having two halves701-1 and 701-2, poled in opposite directions. For example, a first halfSCC 701-1 coupled to electrode 751-1 may be poled in a first direction,and a second half SCC 701-2 coupled to electrode 751-2 may be poled in asecond direction opposite to the first direction. SCC may support asplit polarization without significant artifacts due to the separationbetween individual pillars provided by the polymer matrix. According tosome embodiments, cable 734-1 may couple electrode 751-1 to a voltagesupply at a first voltage. Also, cable 734-2 may couple electrode 751-2to a voltage supply at a second voltage, higher than the first voltage.When the two back electrodes are excited with equal and oppositesignals, the front electrode remains at virtual ground by symmetry, andeach of transducer halves 701-1 and 701-2 receive equal and oppositeelectrical excitation. Electric field 761 is opposite in direction toelectric field 762. Likewise, the polarization induced in SCC 701-1 byelectric field 761 is opposite to the polarization induced in SCC 701-2by electric field 762. Since SCC 701-1 and SCC 701-2 are poled inopposite directions, the piezo-electric effect on first half 701-1 isthe same as the piezo-electric effect on second half 701-2. Thus, anacoustic wave-front including the two halves of split electrodetransducer 750 is generated. Accordingly, in some embodiments halves701-1 and 701-2 vibrate in phase with one another, providing a fullaperture beam.

A single crystal composite as disclosed herein is particularly wellsuited to the split back electrode configuration. Fringe fields at theboundary between the split electrodes 751-1 and 751-2 are mitigated bypolymer matrix 330. This ensures that poling of halves 701-1 and 701-2provides a well-defined orientation near their border.

Some embodiments using ultrasound transducer 750 including a splitelectrode may yield a lower capacitance (higher impedance) device.Indeed, each of the two capacitors formed between electrode 751-1, 752,and 751-2 has a lower capacitance than a capacitor made of the same SCC701 material and having the same thickness, but double the area.Furthermore, in the split electrode configuration the two capacitorsformed between electrodes 751-1, 752, and 751-2 are connected in series,thus reducing the net capacitance of SCC 701 as compared to aconfiguration where back electrodes 751-1 and 751-2 form a singleelectrode. Thus, embodiments of SCC 701 having a split back electrodemay use a higher excitation voltage to achieve the same ultrasoundoutput as a conventional electrode. Embodiments consistent with thesplit electrode configuration illustrated in FIG. 7A provide desirablemanufacture features, since front electrode 752 is floating and may notuse a direct connection to a voltage source, or ground. This simplifiesthe configuration and manufacturing of ultrasound transducer 750 and tiphousing 116. For example, an impedance matching layer such as layer 310(cf. FIG. 3) may be formed as a continuous layer on top of frontelectrode 752.

Split back electrode transducer 750 is desirable in embodimentsincluding matching layer 310. The use of a split back electrode permitsmatching layer 310 to be formed at the wafer level fabrication oftransducer 750 without having a conductive material making contact withfront electrode 752. Thus, fabrication methods according to someembodiments may avoid cutting a hole in matching layer 310 for a frontelectrode contact.

FIG. 7B shows a partial plan view of ultrasound transducer 750 accordingto some embodiments disclosed herein. FIG. 7B illustrates backelectrodes 751-1 and 751-2. FIG. 7B also illustrates molded tip 148 (cf.FIG. 4). In some embodiments electrodes 751-1 and 751-2 in the distalarea close to the tip of molded tip 748 may include a gold plateddiamond grit. Bond pads 761-1 and 761-2 provide electrical contact toelectrodes 751-1 and 751-2 from electrical cables such as cables 134-1and 134-2 (cf. FIG. 5A). Such configuration ensures efficient andreliable electrical contact to SCC 701. Bond pads 761-1 and 761-2 may beformed of any conductive material, like gold or silver. One of ordinaryskill would recognize that the specific material forming bond pads 761-1and 761-2 is not limiting and any conductive material or alloy thereofmay be used, without limitation.

In embodiments using a gold plated diamond grit, SCC 701 is pressed andglued onto molded tip 148. Thus, protuberances in the diamond grit pokeinto the electrode plating on the back of the sheet formed by SCC 701,providing a low resistance electrical connection. Some embodiments mayinclude anisotropic conductive adhesives to provide a reliableelectrical connection to SCC 701. For example, an insulating epoxy-likematerial filled with gold or silver spheres provides an anisotropicconductive adhesive in some implementations. In such embodiments thedensity of the conductive spheres is low enough that the material isnon-conductive, but when the material is compressed into a thin filmbetween two conductive surfaces, the spheres are squished between theconductors and they bridge the narrow gap to again form a low resistanceconnection along the compression direction.

FIG. 7C illustrates front electrode 752, which may be the commonelectrode for SCC transducer 750. In some embodiments electrode 752includes alignment tab 770 to orient the device properly within moldedtip 148. The SCC may include an epoxy matching layer. An acousticimpedance approximately equal to 3 is desirable.

According to some embodiments, SCC 701 including electrodes 752, 751-1,and 751-2 is glued into molded tip 148 forming a dish-shape forproviding focused beam 130 (cf. FIG. 4).

FIGS. 8A-F show a partial view of fabrication stages for an SCC 801,according to some embodiments. FIG. 8A illustrates single crystalmaterial 802 formed into a slab of material 801A, patterned usingphotolithography and DRIE (or other suitable etching and/or materialremoval processes) to etch away portions 825 of material. SCC material802 may be any single crystal, piezo-electric material. For example,some embodiments may use a single crystal including lead magnesiumniobate-lead titanate (PMN-PT). Slab 801A may be formed on a wafer,having a front surface (top of FIG. 8A) and a back surface (bottom ofFIG. 8B). This leads to a slab of material 801B having isolated pillarsor ribs 820, partially formed through the wafer, as illustrated in FIG.8B. In some embodiments a pattern of trenches is etched in thepiezo-electric substrate using DRIE to produce vertical walls(Y-direction) and a very precise geometry (XZ plane), typically with 1μm resolution. After etching, the trenches are filled with a polymer 830such as epoxy or silicone, as illustrated in FIG. 8B.

FIG. 8C illustrates the forming of slab 801C, according to someembodiments. Polymer layer 830 may be on the front side of theultrasound transducer in slab 801B, and material 802 may be on the backside of slab 801B. In some embodiments polymer layer 830 may bepolished, ground, or etched to a thickness such that polymer layer 830and pillars 830 have an edge on the front side of SCC 801.

Thus, slab 801C includes pillars 820 of a piezo material, isolated fromone another on the front side (top of FIG. 8C), contained within polymermatrix 830. The flexibility of slab 801C is adjustable based on the sizeof the trenches formed in the DRIE step and the properties of thepolymer used in matrix 830. Further, slabs 801C may have differentgeometries obtained by photolithography and DRIE steps, as describedabove. In some embodiments, the pattern of pillars 820 may be isolatedislands separated by large moats.

FIG. 8D illustrates forming of slab 801D, including a front electrode852. Forming slab 801D may include forming the SCC layer into a desiredthickness. To accomplish this, material 802 in the back side of slab801C (bottom of FIG. 8C) may be polished, ground, or etched to athickness such that polymer matrix 830 and pillars 820 have an edge onthe back side of SCC 801D. When the substrate is thinned to form acomposite sheet having pillars 820 embedded in polymer matrix 830,individual transducer elements forming an aperture can be selected bytracing a desired outline and removing polymer matrix 830. In someembodiments, tracing the desired outline of individual elements andremoving the polymer may be performed using a laser. The individualtransducer elements are then electroplated to form a front electrode 852in slab 801D in some instances. Front electrode 852 is formed byelectroplating a conductive material on the top portion of slab 801D insome implementations. In some embodiments front electrode 852 andmatching layer 810 are formed while the structure is part of the singlewafer. The thickness of the structure may be 50 μm, 40 μm, 30 μm, orless. In some embodiments, the epoxy layer may be ground to form animpedance matching layer having a ¼ wavelength thickness (orapproximately 15 μm in epoxy).

FIG. 8E illustrates the forming of a back electrode 851 in slab 801E.Back electrode 851 and front electrode 852 may be as electrodes 151 and152 described in detail above (cf. FIG. 3). Back electrode 851 may beformed in the same way as front electrode 852 (cf. FIG. 8D). One of theadvantages of SCC slab 801E is that it has relatively low acousticimpedance, so it can provide a broad frequency response even without anacoustic matching layer.

FIG. 8F illustrates slab SCC 801 formed by depositing an acousticimpedance matching layer 810 on top of slab 801E. Acoustic matchinglayer 810 is included in some embodiments of SCC 801 to match theacoustic impedance of the vessel tissue. Thus, acoustic matching layer810 may further broaden the frequency response of an ultrasoundtransducer using SCC 801.

Once a slab of SCC 801 is complete as shown in FIG. 8E or FIG. 8F, itmay be installed in a catheter tip as an ultrasound transducer.According to some embodiments, SCC 801 is pressed into molded tip 148(cf. FIG. 4). Molded tip 148 may include a curved shape to impart acurved shape to SCC 801 and produce a focused acoustic beam 130. Moldedtip 148 may also provide backing impedance to SCC 801 and attachment ofthe transducer to driveshaft 132 (cf. FIG. 4).

According to embodiments of the fabrication method illustrated in FIGS.8A-F, the dimensions of an ultrasound transducer may be defined at thewafer level. Thus, the dimensions of a finished ultrasound transducermay be determined during the formation of slab 801A (e.g.,photolithography step) and slab 801B (e.g., DRIE step). Furthermore, thefinished ultrasound transducer may be segmented into smaller transducersof any desired size and shape. The flexibility of DRIE allows theformation of pillars 820 of arbitrary shape, forming arbitrary patternswithin polymer matrix 830. For example, some pillar cross-sectionsdiscussed herein are more desirable than traditional square pillars.Having pillars 820 embedded in polymer matrix 830 allows the formationof a round transducer that is cut out using laser ablation.

By having flexibility in the layout and pattern design of an ultrasoundtransducer, fabrication methods for SCC layers as disclosed hereinprovide a focused ultrasound beam using a simple electrical coupling tothe transducer. Some embodiments further include a custom electronicchip, such as a micro-electromechanical system (MEMS), to provide moresophisticated acoustic beam manipulation or modulation.

FIG. 9 shows a flow chart for a method 900 of forming an ultrasoundtransducer according to embodiments disclosed herein. Method 900 will bedescribed below in relation to the steps and structures illustrated inFIGS. 8A-F. Reference to the steps and structures in FIG. 8A-F is usedfor illustrative purposes only and is not limiting of the embodiments ofmethod 900 consistent with the general concept expressed in FIG. 9. Oneof ordinary skill would recognize that obvious variations to method 900may be provided, while maintaining the overall concept as describedbelow.

Step 910 includes etching a single crystal according to a pattern formedby lithography, such as in slab 801A (cf. FIG. 8A). In some embodiments,step 910 includes a DRIE procedure. Step 920 includes placing a polymerlayer on the etched single crystal, to form a slab such as slab 801B(cf. FIG. 8B). In some embodiments step 920 includes filling a pillarpattern resulting from the etching step 910 with polymer, which may bean epoxy. Step 930 includes forming the polymer layer to a thickness,such as in slab 801C (cf. FIG. 8C). Step 930 may include lapping thesurface of the wafer to removing excess epoxy, creating a planar surfaceand exposing the pillars. In step 940 an electrode is placed on thefront side of the SCC.

Step 950 includes forming an SCC layer to a thickness, as in slab 801D(cf. FIG. 8D). In some embodiments step 950 includes grinding the backportion of the wafer including slab 801D to release the compositestructure from the wafer. Step 960 includes placing a back electrode toform a slab such as slab 801E (cf. FIG. 8E). According to someembodiments, step 960 may include similar procedures as step 940 toplace front electrode 852 on slab 801D. In some embodiments, slab 801Eis formed with a plurality of individual transducer elements, eachforming an aperture. Step 960 may include cutting individual transducersfrom slab 801E. The cutting process could be made using a laser tocleanly remove epoxy filler 830 surrounding isolated groups of pillars820. Thus, the piezoelectric material in pillars 820 may be left intactin step 960.

Step 970 includes placing an impedance matching layer on one electrode.Step 970 may include grinding the matching layer to a desired thickness.

Step 980 includes placing the SCC material thus formed on a molded tip,such as molded tip 148. Once the individual transducer is available, itcan be pressed into a micro-molded housing that will become the tip ofthe flexible driveshaft in a rotational IVUS catheter. The moldedhousing may include a dish-shaped depression to form the desiredaperture deflection. In some embodiments, step 980 is performed once thefront and back electrodes are in place (steps 940 and 960). Step 980 mayalso include forming bonding pads to bridge the gap between theelectrical leads inside the driveshaft (e.g., a shielded twisted pair)and the split back electrodes of the transducer. Such bonding pads maybe as described in detail above in reference to bond pads 761-1 and761-2 (cf. FIG. 7B). In some embodiments the fabrication process mayinclude a “Cast-In-Can” method to form a transducer on a molded tip. Insome embodiments, the transducer is pressed into the micro-molded tipsubassembly. In some embodiments the transducer is placed on a moldedtip such that acoustic beam 130 is formed in a plane perpendicular tothe longitudinal axis of the catheter (XY plane in FIG. 2). According tosome embodiments, the transducer is placed on a molded tip such thatacoustic beam 130 extends at an oblique angle with respect to thelongitudinal axis (Z-axis) of the catheter.

Embodiments of the present disclosure described above are exemplaryonly. One skilled in the art may recognize various alternativeembodiments from those specifically disclosed. Those alternativeembodiments are also intended to be within the scope of this disclosure.Accordingly, it is appropriate that the appended claims be construedbroadly and in a manner consistent with the present disclosure.

What is claimed is:
 1. An ultrasound transducer for use inintra-vascular ultrasound (IVUS) imaging systems, the transducercomprising: a single crystal composite (SCC) layer; a front electrode ona side of the SCC layer; and a back electrode on the opposite side ofthe SCC layer; wherein: the SCC layer includes pillars made of a singlecrystal piezo-electric material; the pillars are embedded in a polymermatrix; and the SCC layer has a concave upper surface and an opposingconvex lower surface.
 2. The ultrasound transducer of claim 1 furthercomprising an impedance matching layer of a thickness about ¼ wavelengthof a center frequency of an ultrasound signal transmitted by thetransducer.
 3. The ultrasound transducer of claim 1 wherein the pillarshave an aspect ratio of at least
 2. 4. The ultrasound transducer ofclaim 1 wherein the pillars have a thickness of about 50 μm or less. 5.The ultrasound transducer of claim 1 wherein the pillars cover an areaof about 25% of a surface area of the SCC layer.
 6. The ultrasoundtransducer of claim 1 wherein the pillars have a cross section on asurface of the SCC layer with a shape selected from the group consistingof a circle, a square, a rectangle, and a figure from a random pattern.7. The ultrasound transducer of claim 1 wherein the back electrode issplit into two electrodes electrically decoupled from one another.
 8. AnIVUS imaging system, comprising: an elongate device sized and shaped forinsertion into a vessel of a patient, the elongate device including anultrasound transducer that includes: a single crystal composite (SCC)layer; a front electrode; and a back electrode; wherein: the SCC layerincludes pillars made of a single crystal piezo-electric material; thepillars are embedded in a polymer matrix; and the SCC layer has aconcave surface and an opposing convex surface.
 9. The IVUS imagingsystem of claim 8, wherein the ultrasound transducer is disposed withina distal portion of the elongate device and rotatable with respect to abody of the elongate device.
 10. The IVUS imaging system of claim 9,further comprising an actuator coupled to the ultrasound transducer, theactuator configured to rotate the ultrasound transducer relative to thebody of the elongate device.
 11. The IVUS imaging system of claim 10,wherein the actuator includes a motor and is coupled to the ultrasoundtransducer via a drive shaft.
 12. The IVUS imaging system of claim 8,further comprising a control system configured to control activation ofthe ultrasound transducer to facilitate imaging.
 13. The IVUS imagingsystem of claim 12, wherein the control system is further configured toprocess ultrasound echo data received by the ultrasound transducer togenerate a cross-sectional image of the vessel.
 14. The IVUS imagingsystem of claim 12, further comprising a display in communication withthe control system.
 15. The IVUS imaging system of claim 8, wherein theelongate device is a catheter.